1. Field of the Invention
This invention relates to a tendon for prestressing a concrete (hereinafter referred to as "PC") structure in accordance with a pretensioning method, and more particularly, to a tendon made of fiber reinforced plastic (hereinafter referred to as "FRP").
The FRP-made tendon refers to a rod-like material (so-called FRP-made rod) or a linear material obtained by impregnating a fiber roving in the form of a bundle comprising a plurality of arranged single fibers having a diameter of a few microns, with a thermosetting resin, drawing it through a molding die, and heating and hardening it.
Since the FRP-made tendon has excellent properties when subjected to high magnetic fields, the ocean environment and corrosive environments, FRP-made tendons have been used under these conditions.
Generally, the PC tendon which is used for tensionings, must meet several requirements depending on the use of PC structures. For example, in PC structures such as PC girders, sleepers or the like, on which a bending moment mainly exerts, cracks occur due to an increase in load, which is the case where a unit stress of concrete reaches a level in excess of the tensile strength of the concrete). In this case, the width of the cracks should not increase rapidly and the length of crack should not grow rapidly. Moreover, the fixed length of ends at the concrete structure and the PC tendon should be short. The fixed length referred to is the length over which a compressive force in the concrete in the axial direction of the PC tendon from the end of the concrete structure becomes constant. The fixed length is also the length over which the rate of increase of tensile force of the tendon becomes constant. In other words, prestress is introduced as much as possible over the overall length of the concrete structure.
These features of the PC tendon have been achieved, with the PC steel materials generally used in the past, by increasing the bonding strength between the PC tendon and the concrete in several different ways. On way has been to intentionally allow the steel material of a PC tendon to rust, which rusting process provides the PC steel rod or PC steel wire with a surface which has recesses or projections. Another way has been to use a PC of stranded steel wire.
However, it is difficult to use the aforementioned FRP-made tendons for pretensioning without modification, since the tendons have a smooth outer circumferential surface, and the bond properties between plastic and concrete is inferior to the bond between steel and concrete.
2. Description of the Prior Art
In view of the advantage of the aforementioned FRP-made tendon, Japanese Patent Application Laid-Open No. 3417/75 has proposed a method for increasing the bond properties relative to concrete.
In the aforesaid prior art, rugged-portion forming processing is applied to the outer surface of an FRP-made rod to form a tendon. The rugged-portion forming processing illustrated in the patent application includes a process in which a fiber roving, impregnated with a thermosetting resin is spirally wound about the outer circumferential surface of the rod and then hardened. Another process shown is a knurling which is applied directly to the outer circumferential surface of the rod, and the like.
However, the FRP-made rod which is prepared by the rugged-portion forming processing still results by the bonding between concrete and plastic. In other words, the plastic has poor wettability relative to the concrete, which makes it difficult to form a firm bond therebetween. A major aspect is to retain tension introduced into the rod by mere rugged engagement in an axial direction of the rod and by a measure of frictional force.
Accordingly, in the above-described prior art, the tension introduced into the FRP-made rod, in other words, the prestress force imparted to the concrete structure relies upon the strength of bond properties of (spiral) convex portions applied to the outer circumferential surface of the rod with respect to the rod.
Also, in the arrangement wherein knurling processing is directly applied to the outer circumferential surface of the FRP-made rod which thereby forms concave portions, unidirectional fibers in the outer periphery of the rod tend to be cut, said cutting of the fibers considerably lowering the tensile strength resistance of the rod. As described above, the prior art involves several problems.